The Listeria monocytogenes InlC protein interferes with innate immune responses by targeting the I{kappa}B kinase subunit IKK{alpha}

Ubiquitin Ligases in Control: Regulating NLRP3 Inflammasome Activation

Ubiquitin ligases play pivotal roles in the regulation of NLR family pyrin domain containing 3 (NLRP3) inflammasome activation, a critical process in innate immunity and inflammatory responses. This review explores the intricate mechanisms by which various E3 ubiquitin ligases exert both positive and negative influences on NLRP3 inflammasome activity through diverse post-translational modifications. Negative regulation of NLRP3 inflammasome assembly is mediated by several E3 ligases, including F-box and leucine-rich repeat protein 2 (FBXL2), tripartite motif-containing protein 31 (TRIM31), and Casitas B-lineage lymphoma b (Cbl-b), which induce K48-linked ubiquitination of NLRP3, targeting it for proteasomal degradation. Membrane-associated RING-CH 7 (MARCH7) similarly promotes K48-linked ubiquitination leading to autophagic degradation, while RING finger protein (RNF125) induces K63-linked ubiquitination to modulate NLRP3 function. Ariadne homolog 2 (ARIH2) targets the nucleotide-binding domain (NBD) domain of NLRP3, inhibiting its activation, and tripartite motif-containing protein (TRIM65) employs dual K48 and K63-linked ubiquitination to suppress inflammasome assembly. Conversely, Pellino2 exemplifies a positive regulator, promoting NLRP3 inflammasome activation through K63-linked ubiquitination. Additionally, ubiquitin ligases influence other components critical for inflammasome function. TNF receptor-associated factor 3 (TRAF3) mediates K63 polyubiquitination of apoptosis-associated speck-like protein containing a CARD (ASC), facilitating its degradation, while E3 ligases regulate caspase-1 activation and DEAH-box helicase 33 (DHX33)-NLRP3 complex formation through specific ubiquitination events. Beyond direct inflammasome regulation, ubiquitin ligases impact broader innate immune signaling pathways, modulating pattern-recognition receptor responses and dendritic cell maturation. Furthermore, they intricately control NOD1/NOD2 signaling through K63-linked polyubiquitination of receptor-interacting protein 2 (RIP2), crucial for nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and mitogen-activated protein kinase (MAPK) activation. Furthermore, we explore how various pathogens, including bacteria, viruses, and parasites, have evolved sophisticated strategies to hijack the host ubiquitination machinery, manipulating NLRP3 inflammasome activation to evade immune responses. This comprehensive analysis provides insights into the molecular mechanisms underlying inflammasome regulation and their implications for inflammatory diseases, offering potential avenues for therapeutic interventions targeting the NLRP3 inflammasome. In conclusion, ubiquitin ligases emerge as key regulators of NLRP3 inflammasome activation, exhibiting a complex array of functions that finely tune immune responses. Understanding these regulatory mechanisms not only sheds light on fundamental aspects of inflammation but also offers potential therapeutic avenues for inflammatory disorders and infectious diseases.

The PrfA regulon of Listeria monocytogenes is induced by growth in low-oxygen microaerophilic conditions

Listeria monocytogenes is a food-borne pathogen that must adapt to several environments both inside and outside the host. One such environment is the microaerophilic conditions encountered in the host intestine proximal to the mucosal surface. The aim of this study was to investigate the expression of the PrfA regulon in response to microaerophilic growth conditions in the presence of either glucose or glycerol as a carbon source using four transcriptional (Phly, PactA, P/prfA and P/plcA) gene fusions. Further, RNAseq analysis was used to identify global changes in gene expression during growth in microaerophilic conditions. Following microaerophilic growth, there was a PrfA-dependent increase in transcription from the Phly, PactA and P/plcA promoters, indicating that microaerophilic growth induces the PrfA regulon regardless of the carbon source with increased expression of the PrfA, LLO and ActA proteins. A sigB mutation had no effect on the induction of the PrfA regulon under microaerophilic conditions when glucose was used as a carbon source. In contrast, when glycerol was the carbon source, a sigB mutation increased expression from the Phly and PactA promoters regardless of the level of oxygen. The RNAseq analysis showed that 273 genes were specifically regulated by microaerophilic conditions either up or down including the PrfA regulon virulence factors. Overall, these data indicated that L. monocytogenes PrfA regulon is highly responsive to the low-oxygen conditions likely to be encountered in the small intestine and that SigB has an input into the regulation of the PrfA regulon when glycerol is the sole carbon source.

Listeria in Pregnancy-The Forgotten Culprit

Listeria monocytogenes is a Gram-positive bacterium that causes listeriosis, a severe foodborne illness that is particularly dangerous during pregnancy. It thrives in diverse environments, including refrigerated conditions and food production facilities, due to its adaptability to varying temperatures, pH levels, and salt concentrations. Its virulence stems from the ability to invade host cells, particularly macrophages and epithelial cells, and avoid, or at least postpone, immune detection by utilizing virulence factors such as internalins, listeriolysin O, and actin assembly-inducing protein. This intracellular motility and biofilm formation make LM a persistent pathogen in food safety and public health. Pregnant women are at a much higher risk of listeriosis, which can result in serious fetal complications such as miscarriage, stillbirth, and preterm labor due to LM's affinity for placental tissues. The vertical transmission of LM from mother to fetus can lead to neonatal listeriosis, which can result in sepsis and meningitis, with high mortality rates if not promptly treated. Early diagnosis and treatment with antibiotics, such as ampicillin or gentamicin, are crucial for maternal and neonatal outcomes.

Diverse Listeria monocytogenes in-house clones are present in a dynamic frozen vegetable processing environment

Listeria (L.) monocytogenes is of global concern for food safety as the listeriosis-causing pathogen is widely distributed in the food processing environments, where it can survive for a long time. Frozen vegetables contaminated with L. monocytogenes were recently identified as the source of two large listeriosis outbreaks in the EU and US. So far, only a few studies have investigated the occurrence and behavior of Listeria in frozen vegetables and the associated processing environment. This study investigates the occurrence of L. monocytogenes and other Listeria spp. in a frozen vegetable processing environment and in frozen vegetable products. Using whole genome sequencing (WGS), the distribution of sequence types (MLST-STs) and core genome sequence types (cgMLST-CT) of L. monocytogenes were assessed, and in-house clones were identified. Comparative genomic analyses and phenotypical characterization of the different MLST-STs and isolates were performed, including growth ability under low temperatures, as well as survival of freeze-thaw cycles. Listeria were widely disseminated in the processing environment and five in-house clones namely ST451-CT4117, ST20-CT3737, ST8-CT1349, ST8-CT6243, ST224-CT5623 were identified among L. monocytogenes isolates present in environmental swab samples. Subsequently, the identified in-house clones were also detected in product samples. Conveyor belts were a major source of contamination in the processing environment. A wide repertoire of stress resistance markers supported the colonization and survival of L. monocytogenes in the frozen vegetable processing facility. The presence of ArgB was significantly associated with in-house clones. Significant differences were also observed in the growth rate between different MLST-STs at low temperatures (4 °C and 10 °C), but not between in-house and non-in-house isolates. All isolates harbored major virulence genes such as full length InlA and InlB and LIPI-1, yet there were differences between MLST-STs in the genomic content. The results of this study demonstrate that WGS is a strong tool for tracing contamination sources and transmission routes, and for identifying in-house clones. Further research targeting the co-occurring microbiota and the presence of biofilms is needed to fully understand the mechanism of colonization and persistence in a food processing environment.

Raising a Bacterium to the Rank of a Model System: The Listeria Paradigm

My scientific career has resulted from key decisions and reorientations, sometimes taken rapidly but not always, guided by discussions or collaborations with amazing individuals from whom I learnt a lot scientifically and humanly. I had never anticipated that I would accomplish so much in what appeared as terra incognita when I started to interrogate the mechanisms underlying the virulence of the bacterium Listeria monocytogenes. All this has been possible thanks to a number of talented team members who ultimately became friends.

Human Listeriosis

Listeria monocytogenes is a Gram-positive facultative intracellular pathogen that can cause severe invasive infections upon ingestion with contaminated food. Clinically, listerial disease, or listeriosis, most often presents as bacteremia, meningitis or meningoencephalitis, and pregnancy-associated infections manifesting as miscarriage or neonatal sepsis. Invasive listeriosis is life-threatening and a main cause of foodborne illness leading to hospital admissions in Western countries. Sources of contamination can be identified through international surveillance systems for foodborne bacteria and strains' genetic data sharing. Large-scale whole genome studies have increased our knowledge on the diversity and evolution of L. monocytogenes, while recent pathophysiological investigations have improved our mechanistic understanding of listeriosis. In this article, we present an overview of human listeriosis with particular focus on relevant features of the causative bacterium, epidemiology, risk groups, pathogenesis, clinical manifestations, and treatment and prevention.

Biological Characteristics of Listeria monocytogenes Following Deletion of TatD-like Protein Gene

TatD is the subunit of the twin-arginine translocation (Tat) pathway. Members of TatD family are multifunctional, conserved and widely presented proteins in most prokaryotes. It has been reported that Tat can affect bacterial motility in some bacteria. This study was conducted to determine the contribution of the TatD protein (herein named LmTatD) to the regulation of flagella in Listeria monocytogenes. We constructed an LmTatD gene mutant in L. monocytogenes strain 10403 s and evaluated its biological characteristics. The results showed no difference in growth or morphology between the wild-type strain and the ΔLmTatD mutant. Intriguingly, the ΔLmTatD mutant showed impaired swimming motility and flagella structure but increased biofilm formation. Comparative proteomic analysis using tandem mass tag (TMT) combined with liquid chromatography-tandem mass spectrometry (LC‒MS/MS) was performed to determine differentially expressed proteins (DEPs). The results revealed that 134 proteins out of 2228 total proteins identified were differentially expressed, among which 18 proteins were upregulated and 116 proteins were downregulated in the ΔLmTatD mutant. Analysis of DEPs indicated that the reduced expression levels of the proteins related to flagellar assembly in the ΔLmTatD mutant correlate with its characteristics. Compared to the wild-type strain, the most downregulated proteins in the ΔLmTatD mutant included FlaA, FliD, FliR, FlgD, FlgL, and FlgG. Collectively, our data suggest that although LmTatD is not required for growth in L. monocytogenes, loss of LmTatD reduces flagellar production and motility by regulating flagellar assembly-related protein expression.

Listeria InlB Expedites Vacuole Escape and Intracellular Proliferation by Promoting Rab7 Recruitment via Vps34

Rapid phagosomal escape mediated by listeriolysin O (LLO) is a prerequisite for Listeria monocytogenes intracellular replication and pathogenesis. Escape takes place within minutes after internalization from vacuoles that are negative to the early endosomal Rab5 GTPase and positive to the late endosomal Rab7. Using mutant analysis, we found that the listerial invasin InlB was required for optimal intracellular proliferation of L. monocytogenes. Starting from this observation, we determined in HeLa cells that InlB promotes early phagosomal escape and efficient Rab7 acquisition by the Listeria-containing vacuole (LCV). Recruitment of the class III phosphoinositide 3-kinase (PI3K) Vps34 to the LCV and accumulation of its lipid product, phosphatidylinositol 3-phosphate (PI3P), two key endosomal maturation mediators, were also dependent on InlB. Small interfering RNA (siRNA) knockdown experiments showed that Vps34 was required for Rab7 recruitment and early (LLO-mediated) escape and supported InlB-dependent intracellular proliferation. Together, our data indicate that InlB accelerates LCV conversion into an escape-favorable Rab7 late phagosome via subversion of class III PI3K/Vps34 signaling. Our findings uncover a new function for the InlB invasin in Listeria pathogenesis as an intracellular proliferation-promoting virulence factor. IMPORTANCE Avoidance of lysosomal killing by manipulation of the endosomal compartment is a virulence mechanism assumed to be largely restricted to intravacuolar intracellular pathogens. Our findings are important because they show that cytosolic pathogens like L. monocytogenes, which rapidly escape the phagosome after internalization, can also extensively subvert endocytic trafficking as part of their survival strategy. They also clarify that, instead of delaying phagosome maturation (to allow time for LLO-dependent disruption, as currently thought), via InlB L. monocytogenes appears to facilitate the rapid conversion of the phagocytic vacuole into an escape-conducive late phagosome. Our data highlight the multifunctionality of bacterial virulence factors. At the cell surface, the InlB invasin induces receptor-mediated phagocytosis via class I PI3K activation, whereas after internalization it exploits class III PI3K (Vsp34) to promote intracellular survival. Systematically elucidating the mechanisms by which Listeria interferes with PI3K signaling all along the endocytic pathway may lead to novel anti-infective therapies.

IκBNS-deficiency protects mice from fatal Listeria monocytogenes infection by blunting pro-inflammatory signature in Ly6Chigh monocytes and preventing exaggerated innate immune responses

IκB proteins regulate the inhibition and activation of NF-κB transcription factor complexes. While classical IκB proteins keep NF-κB complexes inactive in the cytoplasm, atypical IκB proteins act on activated NF-κB complexes located in the nucleus. Most of the knowledge regarding the function of IκB proteins has been collected in vitro, while far less is known regarding their impact on activation and regulation of immune responses during in vivo infections. Combining in vivo Listeria monocytogenes (Lm) infection with comparative ex vivo transcriptional profiling of the hepatic response to the pathogen we observed that in contrast to wild type mice that mounted a robust inflammatory response, IκB_NS-deficiency was generally associated with a transcriptional repression of innate immune responses. Whole tissue transcriptomics revealed a pronounced IκB_NS-dependent reduction of myeloid cell-associated transcripts in the liver together with an exceptionally high Nfkbid promoter activity uncovered in Ly6C^high inflammatory monocytes prompted us to further characterize the specific contribution of IκB_NS in the inflammatory response of monocytes to the infectious agent. Indeed, Ly6C^high monocytes primed during Lm infection in the absence of IκB_NS displayed a blunted response compared to wild type-derived Ly6C^high monocytes as evidenced by the reduced early expression of hallmark transcripts of monocyte-driven inflammation such as Il6, Nos2 and Il1β. Strikingly, altered monocyte activation in IκB_NS-deficient mice was associated with an exceptional resistance against Lm infection and protection was associated with a strong reduction in immunopathology in Lm target organs. Of note, mice lacking IκB_NS exclusively in myeloid cells failed to resist Lm infection, indicating that the observed effect was not monocyte intrinsic but monocyte extrinsic. While serum cytokine-profiling did not discover obvious differences between wild type and IκB_NS ^-/- mice for most of the analyzed mediators, IL-10 was virtually undetectable in IκB_NS-deficient mice, both in the steady state and following Lm infection. Together, we show here a crucial role for IκB_NS during Lm infection with IκB_NS-deficient mice showing an overall blunted pro-inflammatory immune response attributed to a reduced pro-inflammatory signature in Ly6C^high monocytes. Reduced immunopathology and complete protection of mice against an otherwise fatal Lm infection identified IκB_NS as molecular driver of inflammation in listeriosis.

A bacterial virulence factor interacts with the splicing factor RBM5 and stimulates formation of nuclear RBM5 granules

L. monocytogenes causes listeriosis, a foodborne disease that is particularly dangerous for immunocompromised individuals and fetuses. Several virulence factors of this bacterial pathogen belong to a family of leucine-rich repeat (LRR)-containing proteins called internalins. Among these, InlP is known for its role in placental infection. We report here a function of InlP in mammalian cell nucleus organization. We demonstrate that bacteria do not produce InlP under in vitro culture conditions. When ectopically expressed in human cells, InlP translocates into the nucleus and changes the morphology of nuclear speckles, which are membrane-less organelles storing splicing factors. Using yeast two-hybrid screen, immunoprecipitation and pull-down experiments, we identify the tumor suppressor and splicing factor RBM5 as a major nuclear target of InlP. InlP inhibits RBM5-induced cell death and stimulate the formation of RBM5-induced nuclear granules, where the SC35 speckle protein redistributes. Taken together, these results suggest that InlP acts as a nucleomodulin controlling compartmentalization and function of RBM5 in the nucleus and that L. monocytogenes has developed a mechanism to target the host cell splicing machinery.

Comparative Genomics Reveal the Utilization Ability of Variable Carbohydrates as Key Genetic Features of Listeria Pathogens in Their Pathogenic Lifestyles

BACKGROUND

L. monocytogenes and L. ivanovii, the only two pathogens of Listeria, can survive in various environments, having different pathogenic characteristics. However, the genetic basis of their excellent adaptability and differences in pathogenicity has still not been completely elucidated.

METHODS

We performed a comparative genomic analysis based on 275 L. monocytogenes, 10 L. ivanovii, and 22 non-pathogenic Listeria strains.

RESULTS

Core/pan-genome analysis revealed that 975 gene families were conserved in all the studied strains. Additionally, 204, 242, and 756 gene families existed uniquely in L. monocytogenes, L. ivanovii, and both, respectively. Functional annotation partially verified that these unique gene families were closely related to their adaptability and pathogenicity. Moreover, the protein-protein interaction (PPI) network analysis of these unique gene sets showed that plenty of carbohydrate transport systems and energy metabolism enzymes were clustered in the networks. Interestingly, ethanolamine-metabolic-process-related proteins were significantly enriched in the PPI network of the unique genes of the Listeria pathogens, which can be understood as a determining factor of their pathogenicity.

CONCLUSIONS

The utilization capacity of multiple carbon sources of Listeria pathogens, especially ethanolamine, is the key genetic basis for their ability to adapt to various environments and pathogenic lifestyles.

Bacterial Toxin and Effector Regulation of Intestinal Immune Signaling

The host immune response is highly effective to detect and clear infecting bacterial pathogens. Given the elaborate surveillance systems of the host, it is evident that in order to productively infect a host, the bacteria often coordinate virulence factors to fine-tune the host response during infection. These coordinated events can include either suppressing or activating the signaling pathways that control the immune response and thereby promote bacterial colonization and infection. This review will cover the surveillance and signaling systems for detection of bacteria in the intestine and a sample of the toxins and effectors that have been characterized that cirumvent these signaling pathways. These factors that promote infection and disease progression have also been redirected as tools or therapeutics. Thus, these toxins are enemies deployed to enhance infection, but can also be redeployed as allies to enable research and protect against infection.

inlF Enhances Listeria monocytogenes Early-Stage Infection by Inhibiting the Inflammatory Response

The internalin family proteins, which carry the leucine repeat region structural motif, play diverse roles in Listeria monocytogenes (Lm) infection and pathogenesis. Although Internalin F, encoded by inlF, was identified more than 20 years ago, its role in the Lm anti-inflammatory response remains unknown. Lm serotype 4b isolates are associated with the majority of listeriosis outbreaks, but the function of InlF in these strains is not fully understood. In this study, we aimed to elucidate the role of inlF in modulating the inflammatory response and pathogenesis of the 4b strain Lm NTSN. Strikingly, although inlF was highly expressed at the transcriptional level during infection of five non-phagocytic cell types, it was not involved in adherence or invasion. Conversely, inlF did contributed to Lm adhesion and invasion of macrophages, and dramatically suppressed the expression of pro-inflammatory cytokines interleukin (IL)-1β and tumor necrosis factor (TNF-α). Consistent with the in vitro results, during Lm infection mice, inlF significantly inhibited the expression of IL-1β and IL-6 in the spleen, as well as IL-1β, IL-6, and TNF-α in the liver. More importantly, inlF contributed to Lm colonization in the spleen, liver, and ileum during the early stage of mouse infection via intragastric administration, inducing severe inflammatory injury and histopathologic changes in the late stage. To our knowledge, this is the first report to demonstrate that inlF mediates the inhibition of the pro-inflammatory response and contributes to the colonization and survival of Lm during the early stage of infection in mice. Our research partly explains the high pathogenicity of serovar 4b strains and will lead to new insights into the pathogenesis and immune evasion of Lm.

Pathogenicity and virulence of Listeria monocytogenes: A trip from environmental to medical microbiology

Listeria monocytogenes is a saprophytic gram-positive bacterium, and an opportunistic foodborne pathogen that can produce listeriosis in humans and animals. It has evolved an exceptional ability to adapt to stress conditions encountered in different environments, resulting in a ubiquitous distribution. Because some food preservation methods and disinfection protocols in food-processing environments cannot efficiently prevent contaminations, L. monocytogenes constitutes a threat to human health and a challenge to food safety. In the host, Listeria colonizes the gastrointestinal tract, crosses the intestinal barrier, and disseminates through the blood to target organs. In immunocompromised individuals, the elderly, and pregnant women, the pathogen can cross the blood-brain and placental barriers, leading to neurolisteriosis and materno-fetal listeriosis. Molecular and cell biology studies of infection have proven L. monocytogenes to be a versatile pathogen that deploys unique strategies to invade different cell types, survive and move inside the eukaryotic host cell, and spread from cell to cell. Here, we present the multifaceted Listeria life cycle from a comprehensive perspective. We discuss genetic features of pathogenic Listeria species, analyze factors involved in food contamination, and review bacterial strategies to tolerate stresses encountered both during food processing and along the host's gastrointestinal tract. Then we dissect host-pathogen interactions underlying listerial pathogenesis in mammals from a cell biology and systemic point of view. Finally, we summarize the epidemiology, pathophysiology, and clinical features of listeriosis in humans and animals. This work aims to gather information from different fields crucial for a comprehensive understanding of the pathogenesis of L. monocytogenes.

Categorizing sequences of concern by function to better assess mechanisms of microbial pathogenesis

To identify sequences with a role in microbial pathogenesis, we assessed the adequacy of their annotation by existing controlled vocabularies and sequence databases. Our goal was to regularize descriptions of microbial pathogenesis for improved integration with bioinformatic applications. Here, we review the challenges of annotating sequences for pathogenic activity. We relate the categorization of more than 2,750 sequences of pathogenic microbes through a controlled vocabulary called Functions of Sequences of Concern (FunSoCs). These allow for an ease of description by both humans and machines. We provide a subset of 220 fully annotated sequences in the supplemental material as examples. The use of this compact (∼30 terms), controlled vocabulary has potential benefits for research in microbial genomics, public health, biosecurity, biosurveillance, and the characterization of new and emerging pathogens.

Genomic Determinants of Pathogenicity and Antimicrobial Resistance for 60 Global Listeria monocytogenes Isolates Responsible for Invasive Infections

Listeria monocytogenes remains a significant public health threat, causing invasive listeriosis manifested as septicemia, meningitis, and abortion, with up to 30% of cases having a fatal outcome. Tracking the spread of invasive listeriosis requires an updated knowledge for virulence factors (VFs) and antimicrobial resistance features, which is an essential step toward its clinical diagnosis and treatment. Taking advantage of high-throughput genomic sequencing, we proposed that the differential genes based on the pathogenomic composition could be used to evaluate clinical observations and therapeutic options for listeriosis. Here, we performed the comparative genomic analysis of 60 strains from five continents with a diverse range of sources, representing serotypes 1/2a, 1/2b, 1/2c, and 4b, comprising lineage I and lineage II and including 13 newly contributed Chinese isolates from clinical cases. These strains were associated with globally distributed clonal groups linked with confirmed foodborne listeriosis outbreak and sporadic cases. We found that L. monocytogenes strains from clonal complex (CC) CC8, CC7, CC9, and CC415 carried most of the adherence and invasive genes. Conversely, CC1, CC2, CC4, and CC6 have the least number of adherence and invasive genes. Additionally, Listeria pathogenicity island-1 (LIPI-1), LIPI-2, intracellular survival, surface anchoring, and bile salt resistance genes were detected in all isolates. Importantly, LIPI-3 genes were harbored in CC3, CC224, and ST619 of the Chinese isolates and in CC1, CC4, and CC6 of other worldwide isolates. Notably, Chinese isolates belonging to CC14 carried antibiotic resistance genes (ARGs) against β-lactams (blaTEM-101, blaTEM-105) and macrolide (ermC-15), whereas CC7 and CC8 isolates harbored ARGs against aminoglycoside (aadA10_2, aadA6_1), which may pose a threat to therapeutic efficacy. Phylogenomic analysis showed that CC8, CC7, and CC5 of Chinese isolates, CC8 (Swiss and Italian isolates), and CC5 and CC7 (Canadian isolates) are closely clustered together and belonged to the same CC. Additionally, CC381 and CC29 of Chinese isolates shared the same genomic pattern as CC26 of Swiss isolate and CC37 of Canadian isolate, respectively, indicating strong phylogenomic relation between these isolates. Collectively, this study highlights considerable clonal diversity with well-recognized virulence and antimicrobial-resistant determinants among Chinese and worldwide isolates that stress to design improved strategies for clinical therapies.

Role of internalin proteins in the pathogenesis of Listeria monocytogenes

Listeria monocytogenes is a food-borne bacterium that causes gastroenteritis, meningitis, or abortion. L. monocytogenes induces its internalization (entry) into human cells and either spreads laterally in tissues or transcytoses to traverse anatomical barriers. In this review, we discuss mechanisms by which five structurally related proteins of the "internalin" family of L. monocytogenes (InlA, InlB, InlC, InlF, and InlP) interact with distinct host receptors to promote infection of human cells and/or crossing of the intestinal, blood-brain, or placental barriers. We focus on recent results demonstrating that the internalin proteins InlA, InlB, and InlC exploit exocytic pathways to stimulate transcytosis, entry, or cell-to-cell spread, respectively. We also discuss evidence that InlA-mediated transcytosis contributes to traversal of the intestinal barrier, whereas InlF promotes entry into endothelial cells to breach the blood-brain barrier. InlB also facilitates the crossing of the blood-brain barrier, but does so by extending the longevity of infected monocytes that may subsequently act as a "Trojan horse" to transfer bacteria to the brain. InlA, InlB, and InlP each contribute to fetoplacental infection by targeting syncytiotrophoblast or cytotrophoblast layers of the placenta. This work highlights the diverse functions of internalins and the complex mechanisms by which these structurally related proteins contribute to disease.

Bacterial Biofilms and Their Implications in Pathogenesis and Food Safety

Biofilm formation is an integral part of the microbial life cycle in nature. In food processing environments, bacterial transmissions occur primarily through raw or undercooked foods and by cross-contamination during unsanitary food preparation practices. Foodborne pathogens form biofilms as a survival strategy in various unfavorable environments, which also become a frequent source of recurrent contamination and outbreaks of foodborne illness. Instead of focusing on bacterial biofilm formation and their pathogenicity individually, this review discusses on a molecular level how these two physiological processes are connected in several common foodborne pathogens such as Listeria monocytogenes, Staphylococcus aureus, Salmonella enterica and Escherichia coli. In addition, biofilm formation by Pseudomonas aeruginosa is discussed because it aids the persistence of many foodborne pathogens forming polymicrobial biofilms on food contact surfaces, thus significantly elevating food safety and public health concerns. Furthermore, in-depth analyses of several bacterial molecules with dual functions in biofilm formation and pathogenicity are highlighted.

Immunoglobulin-fold containing bacterial adhesins: molecular and structural perspectives in host tissue colonization and infection

Immunoglobulin (Ig) domains are one of the most widespread protein domains encoded by the human genome and are present in a large array of proteins with diverse biological functions. These Ig domains possess a central structure, the immunoglobulin-fold, which is a sandwich of two β sheets, each made up of anti-parallel β strands, surrounding a central hydrophobic core. Apart from humans, proteins containing Ig-like domains are also distributed in a vast selection of organisms including vertebrates, invertebrates, plants, viruses and bacteria where they execute a wide array of discrete cellular functions. In this review, we have described the key structural deviations of bacterial Ig-folds when compared to the classical eukaryotic Ig-fold. Further, we have comprehensively grouped all the Ig-domain containing adhesins present in both Gram-negative and Gram-positive bacteria. Additionally, we describe the role of these particular adhesins in host tissue attachment, colonization and subsequent infection by both pathogenic and non-pathogenic Escherichia coli as well as other bacterial species. The structural properties of these Ig-domain containing adhesins, along with their interactions with specific Ig-like and non Ig-like binding partners present on the host cell surface have been discussed in detail.

Three decades of listeriology through the prism of technological advances

Decades of breakthroughs resulting from cross feeding of microbiological research and technological innovation have promoted Listeria monocytogenes to the rank of model microorganism to study host-pathogen interactions. The extraordinary capacity of this bacterium to interfere with a vast array of host cellular processes uncovered new concepts in microbiology, cell biology and infection biology. Here, we review technological advances that revealed how bacteria and host interact in space and time at the molecular, cellular, tissue and whole body scales, ultimately revolutionising our understanding of Listeria pathogenesis. With the current bloom of multidisciplinary integrative approaches, Listeria entered a new microbiology era.

Mechanical competition triggered by innate immune signaling drives the collective extrusion of bacterially infected epithelial cells

Intracellular pathogens alter their host cells' mechanics to promote dissemination through tissues. Conversely, host cells may respond to the presence of pathogens by altering their mechanics to limit infection. Here, we monitored epithelial cell monolayers infected with intracellular bacterial pathogens, Listeria monocytogenes or Rickettsia parkeri, over days. Under conditions in which these pathogens trigger innate immune signaling through NF-κB and use actin-based motility to spread non-lytically intercellularly, we found that infected cell domains formed three-dimensional mounds. These mounds resulted from uninfected cells moving toward the infection site, collectively squeezing the softer and less contractile infected cells upward and ejecting them from the monolayer. Bacteria in mounds were less able to spread laterally in the monolayer, limiting the growth of the infection focus, while extruded infected cells underwent cell death. Thus, the coordinated forceful action of uninfected cells actively eliminates large domains of infected cells, consistent with this collective cell response representing an innate immunity-driven process.

Biofilm-isolated Listeria monocytogenes exhibits reduced systemic dissemination at the early (12-24 h) stage of infection in a mouse model

Environmental cues promote microbial biofilm formation and physiological and genetic heterogeneity. In food production facilities, biofilms produced by pathogens are a major source for food contamination; however, the pathogenesis of biofilm-isolated sessile cells is not well understood. We investigated the pathogenesis of sessile Listeria monocytogenes (Lm) using cell culture and mouse models. Lm sessile cells express reduced levels of the lap, inlA, hly, prfA, and sigB and show reduced adhesion, invasion, translocation, and cytotoxicity in the cell culture model than the planktonic cells. Oral challenge of C57BL/6 mice with food, clinical, or murinized-InlA (InlAm) strains reveals that at 12 and 24 h post-infection (hpi), Lm burdens are lower in tissues of mice infected with sessile cells than those infected with planktonic cells. However, these differences are negligible at 48 hpi. Besides, the expressions of inlA and lap mRNA in sessile Lm from intestinal content are about 6.0- and 280-fold higher than the sessle inoculum, respectively, suggesting sessile Lm can still upregulate virulence genes shortly after ingestion (12 h). Similarly, exposure to simulated gastric fluid (SGF, pH 3) and intestinal fluid (SIF, pH 7) for 13 h shows equal reduction in sessile and planktonic cell counts, but induces LAP and InlA expression and pathogenic phenotypes. Our data show that the virulence of biofilm-isolated Lm is temporarily attenuated and can be upregulated in mice during the early stage (12-24 hpi) but fully restored at a later stage (48 hpi) of infection. Our study further demonstrates that in vitro cell culture assay is unreliable; therefore, an animal model is essential for studying the pathogenesis of biofilm-isolated bacteria.

Innate immune responses to Listeria in vivo

Listeria monocytogenes (Lm) is a foodborne bacterial pathogen that causes listeriosis, a severe infection that manifests as bacteremia and meningo-encephalitis mostly in immunocompromised individuals, and maternal-fetal infection. A critical pathogenic determinant of Lm relies on its ability to actively cross the intestinal barrier, disseminate systemically and cross the blood-brain and placental barriers. Here we illustrate how Lm both evades innate immunity, favoring its dissemination in host tissues, and triggers innate immune defenses that participate to its control.

Virulence characterization and comparative genomics of Listeria monocytogenes sequence type 155 strains

BACKGROUND

Listeria (L.) monocytogenes strains show a high diversity regarding stress tolerance and virulence potential. Genome studies have mainly focused on specific sequence types (STs) predominantly associated with either food or human listeriosis. This study focused on the prevalent ST155, showing equal distribution among clinical and food isolates. We evaluated the virulence potential of 20 ST155 strains and performed comparative genomic analysis of 130 ST155 strains isolated from food, food processing environments and human listeriosis cases in different countries and years.

RESULTS

The in vitro virulence assays using human intestinal epithelial Caco2 and hepatocytic HEPG2 cells showed an impaired virulence phenotype for six of the 20 selected ST155 strains. Genome analysis revealed no distinct clustering of strains from the same source category (food, food processing environment, and clinical isolates). All strains harbored an intact inlA and inlB locus, except four strains, which had an internal deletion in the inlA gene. All strains harbored LIPI-1, but prfA was present in a longer variant in six strains, all showing impaired virulence. The longer PrfA variant resulted in lower expression of inlA, inlB, and prfA, and no expression of hly and actA. Regarding stress-related gene content, SSI-1 was present, whereas qacH was absent in all strains. 34.6% of the strains harbored a plasmid. All but one ST155 plasmids showed high conservation and harbored cadA2, bcrABC, and a triphenylmethane reductase.

CONCLUSIONS

This study contributes to an enhanced understanding of L. monocytogenes ST155 strains, being equally distributed among isolates from humans, food, and food processing environments. The conservation of the present genetic traits and the absence of unique inherent genetic features makes these types of STs especially interesting since they are apparently equally adapted to the conditions in food processing environments, as well as in food as to the human host environment. However, a ST155-specific mutation resulting in a longer PrfA variant impaired the virulence potential of several ST155 strains.

Pathogenic Biohacking: Induction, Modulation and Subversion of Host Transcriptional Responses by Listeria monocytogenes

During infection, the foodborne bacterial pathogen Listeria monocytogenes dynamically influences the gene expression profile of host cells. Infection-induced transcriptional changes are a typical feature of the host-response to bacteria and contribute to the activation of protective genes such as inflammatory cytokines. However, by using specialized virulence factors, bacterial pathogens can target signaling pathways, transcription factors, and epigenetic mechanisms to alter host gene expression, thereby reprogramming the response to infection. Therefore, the transcriptional profile that is established in the host is delicately balanced between antibacterial responses and pathogenesis, where any change in host gene expression might significantly influence the outcome of infection. In this review, we discuss the known transcriptional and epigenetic processes that are engaged during Listeria monocytogenes infection, the virulence factors that can remodel them, and the impact these processes have on the outcome of infection.

Ubiquitination of Listeria Virulence Factor InlC Contributes to the Host Response to Infection

The pathogenic potential of Listeria monocytogenes relies on the production of an arsenal of virulence determinants that have been extensively characterized, including surface and secreted proteins of the internalin family. We have previously shown that the Listeria secreted internalin InlC interacts with IκB kinase α to interfere with the host immune response (E. Gouin, M. Adib-Conquy, D. Balestrino, M.-A. Nahori, et al., Proc Natl Acad Sci USA, 107:17333–17338, 2010, https://doi.org/10.1073/pnas.1007765107). In the present work, we report that InlC is monoubiquitinated on K²²⁴ upon infection of cells and provide evidence that ubiquitinated InlC interacts with and stabilizes the alarmin S100A9, which is a critical regulator of the immune response and inflammatory processes. Additionally, we show that ubiquitination of InlC causes an increase in reactive oxygen species production by neutrophils in mice and restricts Listeria infection. These findings are the first to identify a posttranscriptional modification of an internalin contributing to host defense.

Listeria monocytogenes is a pathogenic bacterium causing potentially fatal foodborne infections in humans and animals. While the mechanisms used by Listeria to manipulate its host have been thoroughly characterized, how the host controls bacterial virulence factors remains to be extensively deciphered. Here, we found that the secreted Listeria virulence protein InlC is monoubiquitinated by the host cell machinery on K224, restricting infection. We show that the ubiquitinated form of InlC interacts with the intracellular alarmin S100A9, resulting in its stabilization and in increased reactive oxygen species production by neutrophils in infected mice. Collectively, our results suggest that posttranslational modification of InlC exacerbates the host response upon Listeria infection.

Dendropanax morbifera Branch Water Extract Increases the Immunostimulatory Activity of RAW264.7 Macrophages and Primary Mouse Splenocytes

Polyacetylenes in the bark of Dendropanax morbifera trees have been reported to promote immune cell proliferation and to strengthen the innate immune system. The immunomodulatory potential of D. morbifera branch water extract (DBW) was evaluated by determining its effect on cell viability and the expression of cytokines and immune effector molecules in mouse RAW264.7 macrophages and splenocytes. Production of nitric oxide (NO), inducible nitric oxide synthase (iNOS), and cytokines (interleukin [IL]-1β, IL-2, and IFN-γ) in RAW264.7 macrophages increased after treatment with DBW. The activation of components of the NF-κB signaling pathway, including the phospho-IκBα and the expression and translocation of p65, a subunit of NF-κB, were also increased in RAW264.7 mouse macrophage cells after treatment with DBW. In addition, when mice were orally administered DBW, splenocyte cytokines and NO production were increased in a dose-dependent manner relative to control-treated mice. Furthermore, natural killer cell activity in DBW-treated mice was determined by lactate dehydrogenase (LDH) release assay. LDH release also increased in response to DBW treatment. Taken together, these results indicate that D. morbifera extract enhances innate immunity by promoting NF-κB signaling, leading to increased expression of proinflammatory cytokines and effector molecules. DBW therefore has potential therapeutic use in the context of immune stimulation.

Virulence Factors of Meningitis-Causing Bacteria: Enabling Brain Entry across the Blood-Brain Barrier

Infections of the central nervous system (CNS) are still a major cause of morbidity and mortality worldwide. Traversal of the barriers protecting the brain by pathogens is a prerequisite for the development of meningitis. Bacteria have developed a variety of different strategies to cross these barriers and reach the CNS. To this end, they use a variety of different virulence factors that enable them to attach to and traverse these barriers. These virulence factors mediate adhesion to and invasion into host cells, intracellular survival, induction of host cell signaling and inflammatory response, and affect barrier function. While some of these mechanisms differ, others are shared by multiple pathogens. Further understanding of these processes, with special emphasis on the difference between the blood-brain barrier and the blood-cerebrospinal fluid barrier, as well as virulence factors used by the pathogens, is still needed.

Selective Autophagy and Xenophagy in Infection and Disease

Autophagy, a cellular homeostatic process, which ensures cellular survival under various stress conditions, has catapulted to the forefront of innate defense mechanisms during intracellular infections. The ability of autophagy to tag and target intracellular pathogens toward lysosomal degradation is central to this key defense function. However, studies involving the role and regulation of autophagy during intracellular infections largely tend to ignore the housekeeping function of autophagy. A growing number of evidences now suggest that the housekeeping function of autophagy, rather than the direct pathogen degradation function, may play a decisive role to determine the outcome of infection and immunological balance. We discuss herein the studies that establish the homeostatic and anti-inflammatory function of autophagy, as well as role of bacterial effectors in modulating and coopting these functions. Given that the core autophagy machinery remains largely the same across diverse cargos, how selectivity plays out during intracellular infection remains intriguing. We explore here, the contrasting role of autophagy adaptors being both selective as well as pleotropic in functions and discuss whether E3 ligases could bring in the specificity to cargo selectivity.

Listeria monocytogenes: cell biology of invasion and intracellular growth

The Gram-positive pathogen Listeria monocytogenes is able to promote its entry into a diverse range of mammalian host cells by triggering plasma membrane remodeling, leading to bacterial engulfment. Upon cell invasion, L. monocytogenes disrupts its internalization vacuole and translocates to the cytoplasm, where bacterial replication takes place. Subsequently, L. monocytogenes uses an actin-based motility system that allows bacterial cytoplasmic movement and cell-to-cell spread. L. monocytogenes therefore subverts host cell receptors, organelles and the cytoskeleton at different infection steps, manipulating diverse cellular functions that include ion transport, membrane trafficking, post-translational modifications, phosphoinositide production, innate immune responses as well as gene expression and DNA stability.

Mechanisms of Blood Brain Barrier Disruption by Different Types of Bacteria, and Bacterial-Host Interactions Facilitate the Bacterial Pathogen Invading the Brain

This review aims to elucidate the different mechanisms of blood brain barrier (BBB) disruption that may occur due to invasion by different types of bacteria, as well as to show the bacteria-host interactions that assist the bacterial pathogen in invading the brain. For example, platelet-activating factor receptor (PAFR) is responsible for brain invasion during the adhesion of pneumococci to brain endothelial cells, which might lead to brain invasion. Additionally, the major adhesin of the pneumococcal pilus-1, RrgA is able to bind the BBB endothelial receptors: polymeric immunoglobulin receptor (pIgR) and platelet endothelial cell adhesion molecule (PECAM-1), thus leading to invasion of the brain. Moreover, Streptococcus pneumoniae choline binding protein A (CbpA) targets the common carboxy-terminal domain of the laminin receptor (LR) establishing initial contact with brain endothelium that might result in BBB invasion. Furthermore, BBB disruption may occur by S. pneumoniae penetration through increasing in pro-inflammatory markers and endothelial permeability. In contrast, adhesion, invasion, and translocation through or between endothelial cells can be done by S. pneumoniae without any disruption to the vascular endothelium, upon BBB penetration. Internalins (InlA and InlB) of Listeria monocytogenes interact with its cellular receptors E-cadherin and mesenchymal-epithelial transition (MET) to facilitate invading the brain. L. monocytogenes species activate NF-κB in endothelial cells, encouraging the expression of P- and E-selectin, intercellular adhesion molecule 1 (ICAM-1), and Vascular cell adhesion protein 1 (VCAM-1), as well as IL-6 and IL-8 and monocyte chemoattractant protein-1 (MCP-1), all these markers assist in BBB disruption. Bacillus anthracis species interrupt both adherens junctions (AJs) and tight junctions (TJs), leading to BBB disruption. Brain microvascular endothelial cells (BMECs) permeability and BBB disruption are induced via interendothelial junction proteins reduction as well as up-regulation of IL-1α, IL-1β, IL-6, TNF-α, MCP-1, macrophage inflammatory proteins-1 alpha (MIP1α) markers in Staphylococcus aureus species. Streptococcus agalactiae or Group B Streptococcus toxins (GBS) enhance IL-8 and ICAM-1 as well as nitric oxide (NO) production from endothelial cells via the expression of inducible nitric oxide synthase (iNOS) enhancement, resulting in BBB disruption. While Gram-negative bacteria, Haemophilus influenza OmpP2 is able to target the common carboxy-terminal domain of LR to start initial interaction with brain endothelium, then invade the brain. H. influenza type b (HiB), can induce BBB permeability through TJ disruption. LR and PAFR binding sites have been recognized as common routes of CNS entrance by Neisseria meningitidis. N. meningitidis species also initiate binding to BMECs and induces AJs deformation, as well as inducing specific cleavage of the TJ component occludin through the release of host MMP-8. Escherichia coli bind to BMECs through LR, resulting in IL-6 and IL-8 release and iNOS production, as well as resulting in disassembly of TJs between endothelial cells, facilitating BBB disruption. Therefore, obtaining knowledge of BBB disruption by different types of bacterial species will provide a picture of how the bacteria enter the central nervous system (CNS) which might support the discovery of therapeutic strategies for each bacteria to control and manage infection.

A Multicolor Split-Fluorescent Protein Approach to Visualize Listeria Protein Secretion in Infection

Listeria monocytogenes is an intracellular food-borne pathogen that has evolved to enter mammalian host cells, survive within them, spread from cell to cell, and disseminate throughout the body. A series of secreted virulence proteins from Listeria are responsible for manipulation of host-cell defense mechanisms and adaptation to the intracellular lifestyle. Identifying when and where these virulence proteins are located in live cells over the course of Listeria infection can provide valuable information on the roles these proteins play in defining the host-pathogen interface. These dynamics and protein levels may vary from cell to cell, as bacterial infection is a heterogeneous process both temporally and spatially. No assay to visualize virulence proteins over time in infection with Listeria or other Gram-positive bacteria has been developed. Therefore, we adapted a live, long-term tagging system to visualize a model Listeria protein by fluorescence microscopy on a single-cell level in infection. This system leverages split-fluorescent proteins, in which the last strand of a fluorescent protein (a 16-amino-acid peptide) is genetically fused to the virulence protein of interest. The remainder of the fluorescent protein is produced in the mammalian host cell. Both individual components are nonfluorescent and will bind together and reconstitute fluorescence upon virulence-protein secretion into the host cell. We demonstrate accumulation and distribution within the host cell of the model virulence protein InlC in infection over time. A modular expression platform for InlC visualization was developed. We visualized InlC by tagging it with red and green split-fluorescent proteins and compared usage of a strong constitutive promoter versus the endogenous promoter for InlC production. This split-fluorescent protein approach is versatile and may be used to investigate other Listeria virulence proteins for unique mechanistic insights in infection progression.

Listeria monocytogenes InlP interacts with afadin and facilitates basement membrane crossing

During pregnancy, the placenta protects the fetus against the maternal immune response, as well as bacterial and viral pathogens. Bacterial pathogens that have evolved specific mechanisms of breaching this barrier, such as Listeria monocytogenes, present a unique opportunity for learning how the placenta carries out its protective function. We previously identified the L. monocytogenes protein Internalin P (InlP) as a secreted virulence factor critical for placental infection. Here, we show that InlP, but not the highly similar L. monocytogenes internalin Lmo2027, binds to human afadin (encoded by AF-6), a protein associated with cell-cell junctions. A crystal structure of InlP reveals several unique features, including an extended leucine-rich repeat (LRR) domain with a distinctive Ca2+-binding site. Despite afadin's involvement in the formation of cell-cell junctions, MDCK epithelial cells expressing InlP displayed a decrease in the magnitude of the traction stresses they could exert on deformable substrates, similar to the decrease in traction exhibited by AF-6 knock-out MDCK cells. L. monocytogenes ΔinlP mutants were deficient in their ability to form actin-rich protrusions from the basal face of polarized epithelial monolayers, a necessary step in the crossing of such monolayers (transcytosis). A similar phenotype was observed for bacteria expressing an internal in-frame deletion in inlP (inlP ΔLRR5) that specifically disrupts its interaction with afadin. However, afadin deletion in the host cells did not rescue the transcytosis defect. We conclude that secreted InlP targets cytosolic afadin to specifically promote L. monocytogenes transcytosis across the basal face of epithelial monolayers, which may contribute to the crossing of the basement membrane during placental infection.

Lmo1656 is a secreted virulence factor of Listeria monocytogenes that interacts with the sorting nexin 6-BAR complex

Listeria monocytogenes (Lm) is a facultative intracellular bacterial pathogen and the causative agent of listeriosis, a rare but fatal disease. During infection, Lm can traverse several physiological barriers; it can cross the intestine and placenta barrier and, in immunocompromised individuals, the blood–brain barrier. With the recent plethora of sequenced genomes available for Lm, it is clear that the complete repertoire of genes used by Lm to interact with its host remains to be fully explored. Recently, we focused on secreted Lm proteins because they are likely to interact with host cell components. Here, we investigated a putatively secreted protein of Lm, Lmo1656, that is present in most sequenced strains of Lm but absent in the nonpathogenic species Listeria innocua. lmo1656 gene is predicted to encode a small, positively charged protein. We show that Lmo1656 is secreted by Lm. Furthermore, deletion of the lmo1656 gene (Δlmo1656) attenuates virulence in mice infected orally but not intravenously, suggesting that Lmo1656 plays a role during oral listeriosis. We identified sorting nexin 6 (SNX6), an endosomal sorting component and BAR domain–containing protein, as a host cell interactor of Lmol656. SNX6 colocalizes with WT Lm during the early steps of infection. This colocalization depends on Lmo1656, and RNAi of SNX6 impairs infection in infected tissue culture cells, suggesting that SNX6 is utilized by Lm during infection. Our results reveal that Lmo1656 is a novel secreted virulence factor of Lm that facilitates recruitment of a specific member of the sorting nexin family in the mammalian host.

Listeria Adhesion Protein Induces Intestinal Epithelial Barrier Dysfunction for Bacterial Translocation

Intestinal epithelial cells are the first line of defense against enteric pathogens, yet bacterial pathogens, such as Listeria monocytogenes, can breach this barrier. We show that Listeria adhesion protein (LAP) induces intestinal epithelial barrier dysfunction to promote bacterial translocation. These disruptions are attributed to the production of pro-inflammatory cytokines TNF-α and IL-6, which is observed in mice challenged with WT and isogenic strains lacking the surface invasion protein Internalin A (ΔinlA), but not a lap- mutant. Additionally, upon engagement of its surface receptor Hsp60, LAP activates canonical NF-κB signaling, facilitating myosin light-chain kinase (MLCK)-mediated opening of the epithelial barrier via cellular redistribution of the epithelial junctional proteins claudin-1, occludin, and E-cadherin. Pharmacological inhibition of MLCK or NF-κB in cells or genetic ablation of MLCK in mice prevents mislocalization of junctional proteins and L. monocytogenes translocation. Thus, L. monocytogenes uses LAP to exploit epithelial defenses and cross the intestinal epithelial barrier.

How to rewire the host cell: A home improvement guide for intracellular bacteria

Intracellular bacterial pathogens have developed versatile strategies to generate niches inside the eukaryotic cells that allow them to survive and proliferate. Making a home inside the host offers many advantages; however, intracellular bacteria must also overcome many challenges, such as disarming innate immune signaling and accessing host nutrient supplies. Gaining entry into the cell and avoiding degradation is only the beginning of a successful intracellular lifestyle. To establish these replicative niches, intracellular pathogens secrete various virulence proteins, called effectors, to manipulate host cell signaling pathways and subvert host defense mechanisms. Many effectors mimic host enzymes, whereas others perform entirely novel enzymatic functions. A large volume of work has been done to understand how intracellular bacteria manipulate membrane trafficking pathways. In this review, we focus on how intracellular bacterial pathogens target innate immune signaling, the unfolded protein response, autophagy, and cellular metabolism and exploit these pathways to their advantage. We also discuss how bacterial pathogens can alter host gene expression by directly modifying histones or hijacking the ubiquitination machinery to take control of several host signaling pathways.

OrfX, a Nucleomodulin Required for Listeria monocytogenes Virulence

Listeria monocytogenes is a bacterial pathogen causing severe foodborne infections in humans and animals. Listeria can enter into host cells and survive and multiply therein, due to an arsenal of virulence determinants encoded in different loci on the chromosome. Several key Listeria virulence genes are clustered in Listeria pathogenicity island 1. This important locus also contains orfX (lmo0206), a gene of unknown function. Here, we found that OrfX is a small, secreted protein whose expression is positively regulated by PrfA, the major transcriptional activator of Listeria virulence genes. We provide evidence that OrfX is a virulence factor that dampens the oxidative response of infected macrophages, which contributes to intracellular survival of bacteria. OrfX is targeted to the nucleus and interacts with the regulatory protein RybP. We show that in macrophages, the expression of OrfX decreases the level of RybP, which controls cellular infection. Collectively, these data reveal that Listeria targets RybP and evades macrophage oxidative stress for efficient infection. Altogether, OrfX is after LntA, the second virulence factor acting directly in the nucleus.IMPORTANCEListeria monocytogenes is a model bacterium that has been successfully used over the last 30 years to refine our understanding of the molecular, cellular, and tissular mechanisms of microbial pathogenesis. The major virulence factors of pathogenic Listeria species are located on a single chromosomal locus. Here, we report that the last gene of this locus encodes a small secreted nucleomodulin, OrfX, that is required for bacterial survival within macrophages and in the infected host. This work demonstrates that the production of OrfX contributes to limiting the host innate immune response by dampening the oxidative response of macrophages. We also identify a target of OrfX, RybP, which is an essential pleiotropic regulatory protein of the cell, and uncover its role in host defense. Our data reinforce the view that the secretion of nucleomodulins is an important strategy used by microbial pathogens to promote infection.

Comparative experimental infection of Listeria monocytogenes and Listeria ivanovii in bovine trophoblasts

Listeria monocytogenes is a Gram-positive, facultative intracellular and invasive bacterium that has tropism to the placenta, and causes fetal morbidity and mortality in several mammalian species. While infection with L. monocytogenes and L. ivanovii are known as important causes of abortion and reproductive failure in cattle, the pathogenesis of maternal-fetal listeriosis in this species is poorly known. This study used the bovine chorioallantoic membrane explant model to investigate the kinetics of L. monocytogenes, L. ivanovii, and L. innocua infections in bovine trophoblastic cells for up to 8 h post infection. L. monocytogenes and L. ivanovii were able to invade and multiply in trophoblastic cells without causing cell death or inducing expression of pro-inflammatory genes. Although L. innocua was unable to multiply in bovine trophoblastic cells, it induced transcription of the pro-inflammatory mediator CXCL6. This study demonstrated for the first time the susceptibility of bovine trophoblastic cells to L. monocytogenes and L. ivanovii infection.

Cell-Based Screen Identifies Human Interferon-Stimulated Regulators of Listeria monocytogenes Infection

The type I interferon (IFN) activated transcriptional response is a critical antiviral defense mechanism, yet its role in bacterial pathogenesis remains less well characterized. Using an intracellular pathogen Listeria monocytogenes (Lm) as a model bacterial pathogen, we sought to identify the roles of individual interferon-stimulated genes (ISGs) in context of bacterial infection. Previously, IFN has been implicated in both restricting and promoting Lm growth and immune stimulatory functions in vivo. Here we adapted a gain-of-function flow cytometry based approach to screen a library of more than 350 human ISGs for inhibitors and enhancers of Lm infection. We identify 6 genes, including UNC93B1, MYD88, AQP9, and TRIM14 that potently inhibit Lm infection. These inhibitors act through both transcription-mediated (MYD88) and non-transcriptional mechanisms (TRIM14). Further, we identify and characterize the human high affinity immunoglobulin receptor FcγRIa as an enhancer of Lm internalization. Our results reveal that FcγRIa promotes Lm uptake in the absence of known host Lm internalization receptors (E-cadherin and c-Met) as well as bacterial surface internalins (InlA and InlB). Additionally, FcγRIa-mediated uptake occurs independently of Lm opsonization or canonical FcγRIa signaling. Finally, we established the contribution of FcγRIa to Lm infection in phagocytic cells, thus potentially linking the IFN response to a novel bacterial uptake pathway. Together, these studies provide an experimental and conceptual basis for deciphering the role of IFN in bacterial defense and virulence at single-gene resolution.

While the type I interferon response is known to be activated by both viruses and bacteria, it has mostly been characterized in terms of its antiviral properties. Listeria monocytogenes, an opportunistic Gram-positive bacterial pathogen with up to 50% mortality rate and a variety of clinical manifestations, is a potent activator of interferon secretion. In mouse models, interferon has been previously implicated in both restricting and promoting L. monocytogenes infection. Here, we utilized a high-throughput flow-cytometry based approach to screen a library of human interferon I stimulated genes (ISGs) and identified regulators of L. monocytogenes infection. These include inhibitors that act through both transcriptional (MYD88) and transcription-independent (TRIM14) mechanisms. Strikingly, expression of the human high affinity immunoglobulin receptor FcγRIa (CD64) was found to potently enhance L. monocytogenes infection. Both biochemical and cellular studies indicate that FcγRIa increases primary invasion of L. monocytogenes through a previously uncharacterized IgG-independent internalization mechanism. Together, these studies provide an important insight into the complex role of interferon response in bacterial virulence and host defense.

InlP, a New Virulence Factor with Strong Placental Tropism

Intrauterine infection is a major detriment for maternal-child health and occurs despite local mechanisms that protect the maternal-fetal interface from a wide variety of pathogens. The bacterial pathogen Listeria monocytogenes causes spontaneous abortion, stillbirth, and preterm labor in humans and serves as a model for placental pathogenesis. Given the unique immunological environment of the maternal-fetal interface, we hypothesized that virulence determinants with placental tropism are required for infection of this tissue. We performed a genomic screen in pregnant guinea pigs that led to the identification of 201 listerial genes important for infection of the placenta but not maternal liver. Among these genes was lmrg1778 (lmo2470), here named inlP, predicted to encode a secreted protein that belongs to the internalin family. InlP is conserved in virulent L. monocytogenes strains but absent in Listeria species that are nonpathogenic for humans. The intracellular life cycle of L. monocytogenes deficient in inlP (ΔinlP) was not impaired. In guinea pigs and mice, InlP increased the placental bacterial burden by a factor of 3 log₁₀ while having only a minor role in other maternal organs. Furthermore, the ΔinlP strain was attenuated in intracellular growth in primary human placental organ cultures and trophoblasts. InlP is a novel virulence factor for listeriosis with a strong tropism for the placenta. This virulence factor represents a tool for the development of new modalities to prevent and treat infection-related pregnancy complications.

The ubiquitin system: a critical regulator of innate immunity and pathogen-host interactions

The ubiquitin system comprises enzymes that are responsible for ubiquitination and deubiquitination, as well as ubiquitin receptors that are capable of recognizing and deciphering the ubiquitin code, which act in coordination to regulate almost all host cellular processes, including host-pathogen interactions. In response to pathogen infection, the host innate immune system launches an array of distinct antimicrobial activities encompassing inflammatory signaling, phagosomal maturation, autophagy and apoptosis, all of which are fine-tuned by the ubiquitin system to eradicate the invading pathogens and to reduce concomitant host damage. By contrast, pathogens have evolved a cohort of exquisite strategies to evade host innate immunity by usurping the ubiquitin system for their own benefits. Here, we present recent advances regarding the ubiquitin system-mediated modulation of host-pathogen interplay, with a specific focus on host innate immune defenses and bacterial pathogen immune evasion.

Latex protein extracts from Calotropis procera with immunomodulatory properties protect against experimental infections with Listeria monocytogenes

BACKGROUND

The latex from the medicinal plant Calotropis procera is often used in folk medicine against infectious and inflammatory diseases.

PURPOSE

In this study, we investigate a protein fraction with immunomodulatory properties, named LPPI, against experimental infections, in vitro and in vivo, with a virulent strain of Listeria monocytogenes.

STUDY DESIGN

LPPI was exposed to cultured macrophages or Swiss mice and then challenged with L. monocytogenes.

METHODS

Peritoneal macrophages were obtained from Swiss mice, and cultured in 96-well microplates. Soluble latex proteins (LP) were subjected to fractionation by ion-exchange chromatography. The major peak (LPPI) was added into wells at 10 or 100µg/ml. Albumin (100µg/ml) was used for comparison between protein treatments. After incubation for 1h at 5% CO2/ 37°C, the supernatant was discarded and 0.2ml of L. monocytogenes overnight culture was added in the wells. Following 4h and 24h infection, the cytokine mRNA expression was evaluated as well as the number of intracellular colony forming units. Swiss mice (n=16) were injected intraperitoneally (i.p.) with LPPI (5 and 10mg/kg) while the control mice received albumin (10mg/kg) or LP (10mg/kg). After 24h, all animal groups were challenged with L. monocytogenes (10(6) CFU/ ml), also by i.p. route.

RESULTS

LPPI was not toxic to uninfected macrophages (pMØ) and significantly increased mRNA expression of TNF-α, IL-6, IL-1β and iNOS. Following infection, cell viability was reduced by 50% in albumin-treated pMØ (control); but only 17% in pMØ treated with LPPI at 100µg/ml. In this case, LPPI increased expression of TNF-α and IL-6 whereas the number of bacterial colony-forming units was reduced 100-fold in comparison to control groups. Swiss mice pretreated with LPPI showed dose-dependent survival rates that reached 80%, while mice that received albumin died 1-3 days after infection. After 24h infection, leukocyte migration to the infectious foci was high in LPPI-treated mice whereas the number of viable bacteria in the peritoneal fluid, liver and bloodstream were significantly reduced.

CONCLUSION

We conclude that LPPI present immunomodulatory properties that are beneficial for prevention of systemic bacterial infections caused by the intracellular bacteria L. monocytogenes.

Strategies Used by Bacteria to Grow in Macrophages

Intracellular bacteria are often clinically relevant pathogens that infect virtually every cell type found in host organisms. However, myeloid cells, especially macrophages, constitute the primary cells targeted by most species of intracellular bacteria. Paradoxically, macrophages possess an extensive antimicrobial arsenal and are efficient at killing microbes. In addition to their ability to detect and signal the presence of pathogens, macrophages sequester and digest microorganisms using the phagolysosomal and autophagy pathways or, ultimately, eliminate themselves through the induction of programmed cell death. Consequently, intracellular bacteria influence numerous host processes and deploy sophisticated strategies to replicate within these host cells. Although most intracellular bacteria have a unique intracellular life cycle, these pathogens are broadly categorized into intravacuolar and cytosolic bacteria. Following phagocytosis, intravacuolar bacteria reside in the host endomembrane system and, to some extent, are protected from the host cytosolic innate immune defenses. However, the intravacuolar lifestyle requires the generation and maintenance of unique specialized bacteria-containing vacuoles and involves a complex network of host-pathogen interactions. Conversely, cytosolic bacteria escape the phagolysosomal pathway and thrive in the nutrient-rich cytosol despite the presence of host cell-autonomous defenses. The understanding of host-pathogen interactions involved in the pathogenesis of intracellular bacteria will continue to provide mechanistic insights into basic cellular processes and may lead to the discovery of novel therapeutics targeting infectious and inflammatory diseases.

LMOh7858_0369, a gene encoding a putative leucine-rich repeat-containing protein, is required for virulence of Listeria monocytogenes

Listeria monocytogenes possesses the highest number of leucine-rich repeat (LRR)-containing proteins among all Gram-positive bacteria; these LRR-containing molecules are known as the 'internalin' family. To understand the functions of largely uncharacterized LRR-containing molecules, we constructed seven deletion mutants in the L. monocytogenes H7858 strain targeting genes in this family and tested their virulence. Among the seven mutants, the ΔLMOh7858_0369 strain and the ΔLMOh7858_2546 strain showed significantly impaired invasiveness of HepG2 cells. We further tested the virulence of these two strains in the intravascular sepsis model using BALB/c mice. Interestingly, the ΔLMOh7858_0369 strain showed significant reduction in organ colonization, bacteremia and invasion of the brain compared with the parental wild-type strain. Host immune responses to listerial intravascular infection were measured at 24 and 72 h post-infection. Transcript levels of several proinflammatory cytokines and chemokines were significantly lower when induced by the ΔlmOh7858_0369 strain than when induced by the wild type. These results suggest that the putative LRR-containing protein encoded by LMOh7858_0369 might be a novel virulence factor of the L. monocytogenes H7858 strain.

Pathogen-nematode interaction: Nitrogen supply of Listeria monocytogenes during growth in Caenorhabditis elegans

Listeria monocytogenes is a Gram-positive facultatively intracellular human pathogen. Due to its saprophytic lifestyle, L. monocytogenes is assumed to infect and proliferate within soil organisms such as Caenorhabditis elegans. However, little is known about the nutrient usages and metabolite fluxes in this bacterium-nematode interaction. Here, we established a nematode colonization model for L. monocytogenes and a method for the efficient separation of the pathogen from the nematodal gut. Following (15)N labelling of C. elegans and gas chromatography-mass spectrometry-based (15)N isotopologue analysis, we detected a high basal metabolic rate of the nematode, and observed a significant metabolic flux from nitrogenous compounds of the nematode to listerial proteins during proliferation of the pathogen in the worm's intestine. For comparison, we also measured the N fluxes from the gut content into listerial proteins using completely (15)N-labelled Escherichia coli OP50 as food for C. elegans. In both settings, L. monocytogenes prefers the direct incorporation of histidine, arginine and lysine over their de novo biosynthesis. Our data suggest that colonization of nematodes is a strategy of L. monocytogenes to increase its access to N-rich nutrients.

A detailed view of the intracellular transcriptome of Listeria monocytogenes in murine macrophages using RNA-seq

Listeria monocytogenes is a bacterial pathogen and causative agent for the foodborne infection listeriosis, which is mainly a threat for pregnant, elderly, or immunocompromised individuals. Due to its ability to invade and colonize diverse eukaryotic cell types including cells from invertebrates, L. monocytogenes has become a well-established model organism for intracellular growth. Almost 10 years ago, we and others presented the first whole-genome microarray-based intracellular transcriptome of L. monocytogenes. With the advent of newer technologies addressing transcriptomes in greater detail, we revisit this work, and analyze the intracellular transcriptome of L. monocytogenes during growth in murine macrophages using a deep sequencing based approach. We detected 656 differentially expressed genes of which 367 were upregulated during intracellular growth in macrophages compared to extracellular growth in Brain Heart Infusion broth. This study confirmed ∼64% of all regulated genes previously identified by microarray analysis. Many of the regulated genes that were detected in the current study involve transporters for various metals, ions as well as complex sugars such as mannose. We also report changes in antisense transcription, especially upregulations during intracellular bacterial survival. A notable finding was the detection of regulatory changes for a subset of temperate A118-like prophage genes, thereby shedding light on the transcriptional profile of this bacteriophage during intracellular growth. In total, our study provides an updated genome-wide view of the transcriptional landscape of L. monocytogenes during intracellular growth and represents a rich resource for future detailed analysis.

Listeriosis in animals, its public health significance (food-borne zoonosis) and advances in diagnosis and control: a comprehensive review

Listeriosis is an infectious and fatal disease of animals, birds, fish, crustaceans and humans. It is an important food-borne zoonosis caused by Listeria monocytogenes, an intracellular pathogen with unique potential to spread from cell to cell, thereby crossing blood-brain, intestinal and placental barriers. The organism possesses a pile of virulence factors that help to infect the host and evade from host immune machinery. Though disease occurrence is sporadic throughout the world, it can result in severe damage during an outbreak. Listeriosis is characterized by septicaemia, encephalitis, meningitis, meningoencephalitis, abortion, stillbirth, perinatal infections and gastroenteritis with the incubation period varying with the form of infection. L. monocytogenes has been isolated worldwide from humans, animals, poultry, environmental sources like soil, river, decaying plants, and food sources like milk, meat and their products, seafood and vegetables. Since appropriate vaccines are not available and infection is mainly transmitted through foods in humans and animals, hygienic practices can prevent its spread. The present review describes etiology, epidemiology, transmission, clinical signs, post-mortem lesions, pathogenesis, public health significance, and advances in diagnosis, vaccines and treatment of this disease. Special attention has been given to novel as well as prospective emerging therapies that include bacteriophage and cytokine therapy, avian egg yolk antibodies and herbal therapy. Various vaccines, including advances in recombinant and DNA vaccines and their modes of eliciting immune response, are also discussed. Due focus has also been given regarding appropriate prevention and control strategies to be adapted for better management of this zoonotic disease.